Current and future research is carried out in the following areas: (1) Investigation of possible photorepair in DNA of UV-irradiated human and other mammalian cells, by means of (a) competitive inhibition of the Haemophilus DNA assay system, (b) cell survival and (c) H3-thymidine labeling. (2) Characterization of adverse "white light" (WL) effects (wavelengths approx. 340-400nm), resulting in competitive inhibition of the Haemophilus DNA assay system and thereby masking photorepair or interfering with it. WL effects on mammalian cell may be important by themselves, as the effecting wavelengths are present in natural sunlight and artificial light sources. (3) Overlap of dark repair processes with photorepair and WL effects in mammalian cells. (4) Investigation of differences in photo-damage and repair capacities, comparing different mammalian tissues and donors. BIBLIOGRAPHIC REFERENCES: H. Harm and C. S. Rupert: Analysis of photoenzymatic repair of UV lesions in DNA by single light flashes. XI. Light-induced activation of the yeast photoreactivating enzyme. Mutation Res. 34, 75-92 (1976). H. Harm: Repair of UV-irradiated Biological Systems: Photoreactivation. In: Photochemistry and Photobiology of Nucleic Acids (Ed. S. Y. Wang Academic Press, N. Y. (Feb. 1976).